1. Field of the Invention
The present invention relates generally to an apparatus and method for channel estimation and Cyclic Prefix (CP) reconstruction in an Orthogonal Frequency Division Multiplexing (OFDM) mobile communication system, and in particular, to an apparatus and method for channel estimation and CP reconstruction in an Orthogonal Frequency Division Multiplexing-Space-Time Block Coding (OFDM-STBC) mobile communication system.
2. Description of the Related Art
Today's mobile communication systems are evolving toward high-speed, high-quality mobile communication systems for providing data service and multimedia service beyond the traditional voice service. For provisioning of such a high-speed, high-quality data service, OFDM is considered prominent for 4th generation mobile communication systems due to its excellent resource use efficiency. Orthogonal Frequency Division Multiple Access (OFDMA) or Orthogonal Frequency Division Multiplexing-Frequency Division Multiple Access (OFDM-FDMA) is a major OFDM system in which a plurality of users share a given time using different subchannels.
Meanwhile, factors inherent to a radio channel environment impose constraints on use of high-order modulation schemes and high coding rates for the high-speed, high-quality data service. The factors include white noise, fading-caused change in received signal power, shadowing, Doppler effects caused by the mobility and frequent velocity change of Mobile Stations (MSs), and interference from other users and multipath signals. Accordingly, mobile communication systems adaptively use modulation and coding schemes according to the varying radio channel environments. To do so, accurate channel estimation is essential.
When a channel changes fast as in a fast mobile environment, the change is present even within one OFDM symbol in the OFDM mobile communication system. The resulting loss of orthogonality between subchannels leads to severe Inter-Carrier Interference (ICI) and Inter-Symbol Interference (ISI). The ICI and ISI significantly deteriorate channel estimation performance and thus the use of an error correction code is still not effective in improving the reliability of recovered symbols.
The OFDM system decreases the ISI and ICI generally by increasing a symbol duration in proportion to the number of subcarriers. The most general way is to add a guard interval every predetermined number of transmission symbols. A copy of last few symbols is inserted as a guard interval called a CP. The number of the inserted symbols is usually determined by that of subcarriers. The CP must be longer than a Channel Impulse Response (CIR) representing the characteristics of a radio channel. If the CIR is longer than the CP, the ICI and ISI deteriorate system performance. However, the CP insertion decreases frequency efficiency. That is, the transmission period of data symbols is reduced by as much as the CP length.
Accordingly, a need exists for reducing a CP length and reproducing a shortened CP iteratively in order to minimize the decrease of frequency efficiency. Residual Inter-Symbol Interference Cancellation (RISIC) is a major one of such techniques. The RISIC is a technique of eliminating ICI and ISI components caused by the use of a CP of insufficient length, that is, a CP shorter than a CIR. It involves tail cancellation and cyclicity restoration. The tail cancellation eliminates an ISI signal component from a current symbol using the previous symbol estimate. The cyclicity restoration cancels an ICI signal component from the current symbol by iterative symbol reconstruction and cyclic reconstruction. The RISIC is generalized as shown in Equation (1):
                                                                        r                ~                            k              i                        =                                          r                k                i                            -                                                ∑                                      l                    =                                          G                      +                      k                      +                      1                                                        L                                ⁢                                                      h                    l                                    ⁢                                      x                                                                  (                                                  k                          -                          l                          +                          G                                                )                                            N                                                              i                      -                      1                                                                                  +                                                ∑                                      l                    =                                          G                      +                      k                      +                      1                                                        L                                ⁢                                                      h                    l                                    ⁢                                      x                                                                  (                                                  k                          -                          l                                                )                                            N                                                              i                      -                      1                                                                                                    ,                                          ⁢                      0            ≤            k            <                          L              -              G                                      ⁢                                  ⁢                                  ⁢                                                            Tail                ⁢                                                                  ⁢                Cancellation                                                                    Cyclicity                ⁢                                                                  ⁢                Restoration                                                                                                                              {                                          r                      k                      i                                        }                                                        k                    =                    0                                                        N                    -                    1                                                  :                                                      i                    th                                    ⁢                                                                          ⁢                  received                  ⁢                                                                          ⁢                  symbol                  ⁢                                                                          ⁢                  sequence                                                                                                      h                  l                                :                                                      l                    th                                    ⁢                                                                          ⁢                  tap                  ⁢                                                                          ⁢                  of                  ⁢                                                                          ⁢                  CIR                  ⁢                                                                          ⁢                                      G                    :                                          CP                      ⁢                                                                                          ⁢                      length                                                                                                                                                                                    {                                          x                      k                      i                                        }                                                        k                    =                    0                                                        N                    -                    1                                                  :                                                      i                    th                                    ⁢                                                                          ⁢                  time                  ⁢                                      -                                    ⁢                  domain                  ⁢                                                                          ⁢                  transmitted                  ⁢                                                                          ⁢                  symbol                  ⁢                                                                          ⁢                  sequence                                                                                                                          (                    k                    )                                    N                                :                                  k                  ⁢                                                                          ⁢                  modulo                  ⁢                                                                          ⁢                  N                  ⁢                                                                          ⁢                                      L                    :                                          CIR                      ⁢                                                                                          ⁢                      length                                                                                                                              (        1        )            
As described above, the RISIC eliminates the ISI and ICI signal components by tail cancellation and cyclicity restoration. This RISIC technique requires channel information, and inaccurate channel information may cause error propagation. Therefore, a method for estimating channel information accurately is needed to cancel the ISI and ICI by the RISIC.
Since many existing channel estimation schemes for the OFDM system are based on the premise that a CP is longer than a CIR, they are not feasible for the RISIC and if used, they cause severe performance degradation. Hence, a channel estimation technique for an OFDM system using the RISIC is yet to be developed.
The mobile communication systems use multiple antenna schemes to transmit a large amount of data at a low error probability. One of them is STBC which is a closed-loop scheme operating without feedback channel information.
However, considering that the RISIC was developed for application to an OFDM system using a single transmit antenna, it is not viable for an OFDM-STBC system. Accordingly, there exists a need for a method of achieving antenna diversity gain by applying the RISIC to the OFDM-STBC system.